ArrayLists
8 min · Java
Java's resizable lists: add, remove, and reach items by index.
Enemy 1 of 5
This code makes an ArrayList of Strings, adds apple and banana, then prints the item at position 0. Counting starts at 0. Press Run to see it.
Why this works
Java does have a plain array type, but its size is fixed forever once created, useful in some situations, but inconvenient the moment you don't know in advance how many items you'll end up with. An ArrayList solves that: it holds many values in order, just like an array, but can grow or shrink as your program runs.
To use one, you first import java.util.ArrayList (Java organizes its built-in tools into packages, and import brings a specific one into your file), then new ArrayList<>() creates an empty list. The <String> part is called a generic type parameter, it tells Java exactly what kind of values this particular ArrayList is allowed to hold, so an ArrayList<String> can never accidentally end up holding a number.
.add() appends a new value to the end. .get(index) reads the value at a given position, counting from 0, the same zero-based indexing you'll see in every language's collections, so the first item is index 0, not 1.
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